[Blazor][HotReload] Capture the execution context before calling RenderRootComponentsOnHotReload (#63846)

Captures the execution context at the time a root component is initialized and runs the hot reload callback against that context.

Author: javiercn

src/Components/Components/src/RenderTree/Renderer.cs

@@ -54,6 +54,7 @@ public abstract partial class Renderer : IDisposable, IAsyncDisposable
     private bool _rendererIsDisposed;
 
     private bool _hotReloadInitialized;
+    private HotReloadRenderHandler? _hotReloadRenderHandler;
 
     /// <summary>
     /// Allows the caller to handle exceptions from the SynchronizationContext when one is available.
@@ -231,7 +232,12 @@ protected internal int AssignRootComponentId(IComponent component)
             _hotReloadInitialized = true;
             if (HotReloadManager.MetadataUpdateSupported)
             {
-                HotReloadManager.OnDeltaApplied += RenderRootComponentsOnHotReload;
+                // Capture the current ExecutionContext so AsyncLocal values present during initial root component
+                // registration flow through to hot reload re-renders. Without this, hot reload callbacks execute
+                // on a thread without the original ambient context and AsyncLocal values appear null.
+                var executionContext = ExecutionContext.Capture();
+                _hotReloadRenderHandler = new HotReloadRenderHandler(this, executionContext);
+                HotReloadManager.OnDeltaApplied += _hotReloadRenderHandler.RerenderOnHotReload;
             }
         }
 
@@ -1234,9 +1240,9 @@ protected virtual void Dispose(bool disposing)
             _rendererIsDisposed = true;
         }
 
-        if (_hotReloadInitialized && HotReloadManager.MetadataUpdateSupported)
+        if (_hotReloadInitialized && HotReloadManager.MetadataUpdateSupported && _hotReloadRenderHandler is not null)
         {
-            HotReloadManager.OnDeltaApplied -= RenderRootComponentsOnHotReload;
+            HotReloadManager.OnDeltaApplied -= _hotReloadRenderHandler.RerenderOnHotReload;
         }
 
         // It's important that we handle all exceptions here before reporting any of them.
@@ -1371,4 +1377,19 @@ public async ValueTask DisposeAsync()
             }
         }
     }
+
+    private sealed class HotReloadRenderHandler(Renderer renderer, ExecutionContext? executionContext)
+    {
+        public void RerenderOnHotReload()
+        {
+            if (executionContext is null)
+            {
+                renderer.RenderRootComponentsOnHotReload();
+            }
+            else
+            {
+                ExecutionContext.Run(executionContext, static s => ((Renderer)s!).RenderRootComponentsOnHotReload(), renderer);
+            }
+        }
+    }
 }

src/Components/Components/test/RendererTest.cs

@@ -4,6 +4,7 @@
 using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Globalization;
+using System.Reflection;
 using System.Runtime.ExceptionServices;
 using Microsoft.AspNetCore.Components.CompilerServices;
 using Microsoft.AspNetCore.Components.HotReload;
@@ -5027,6 +5028,40 @@ public async Task DisposingRenderer_UnsubsribesFromHotReloadManager()
         Assert.False(hotReloadManager.IsSubscribedTo);
     }
 
+    [Fact]
+    public async Task HotReload_ReRenderPreservesAsyncLocalValues()
+    {
+        await using var renderer = new TestRenderer();
+
+        var hotReloadManager = new HotReloadManager { MetadataUpdateSupported = true };
+        renderer.HotReloadManager = hotReloadManager;
+        HotReloadManager.Default.MetadataUpdateSupported = true;
+
+        var component = new AsyncLocalCaptureComponent();
+
+        // Establish AsyncLocal value before registering hot reload handler / rendering.
+        ServiceAccessor.TestAsyncLocal.Value = "AmbientValue";
+
+        var componentId = renderer.AssignRootComponentId(component);
+        await renderer.Dispatcher.InvokeAsync(() => renderer.RenderRootComponentAsync(componentId));
+
+        // Sanity: initial render should not have captured a hot-reload value yet.
+        Assert.Null(component.HotReloadValue);
+
+        // Simulate hot reload delta applied from a fresh thread (different ExecutionContext) so the AsyncLocal value is lost.
+        var expected = ServiceAccessor.TestAsyncLocal.Value;
+        var thread = new Thread(() =>
+        {
+            // Simulate environment where the ambient value is not present on the hot reload thread.
+            ServiceAccessor.TestAsyncLocal.Value = null;
+            hotReloadManager.TriggerOnDeltaApplied();
+        });
+        thread.Start();
+        thread.Join();
+
+        Assert.Equal(expected, component.HotReloadValue);
+    }
+
     [Fact]
     public void ThrowsForUnknownRenderMode_OnComponentType()
     {
@@ -5180,6 +5215,34 @@ protected override Task UpdateDisplayAsync(in RenderBatch renderBatch)
             => Task.CompletedTask;
     }
 
+    private class ServiceAccessor
+    {
+        public static AsyncLocal<string> TestAsyncLocal = new AsyncLocal<string>();
+    }
+
+    private class AsyncLocalCaptureComponent : IComponent
+    {
+        private bool _initialized;
+        private RenderHandle _renderHandle;
+        public string HotReloadValue { get; private set; }
+
+        public void Attach(RenderHandle renderHandle) => _renderHandle = renderHandle;
+
+        public Task SetParametersAsync(ParameterView parameters)
+        {
+            if (!_initialized)
+            {
+                _initialized = true; // First (normal) render, don't capture.
+            }
+            else
+            {
+                // Hot reload re-render path.
+                HotReloadValue = ServiceAccessor.TestAsyncLocal.Value;
+            }
+            return Task.CompletedTask;
+        }
+    }
+
     private class TestComponent : IComponent, IDisposable
     {
         private RenderHandle _renderHandle;

src/Components/Shared/src/HotReloadManager.cs

@@ -25,4 +25,7 @@ internal sealed class HotReloadManager
     /// MetadataUpdateHandler event. This is invoked by the hot reload host via reflection.
     /// </summary>
     public static void UpdateApplication(Type[]? _) => Default.OnDeltaApplied?.Invoke();
+
+    // For testing purposes only
+    internal void TriggerOnDeltaApplied() => OnDeltaApplied?.Invoke();
 }